Spatiotemporal Mapping of Efficient Chiral Induction by Helicene‐Type Additives in Copolymer Thin Films

Abstract We observed efficient induction of chirality in polyfluorene copolymer thin films by mixing with helicene‐type chiral additives based on the dibenzo[c,h]acridine motif. Images obtained from circular dichroism (CD) and circularly polarized luminescence (CPL) microscopy provide information about the chiral arrangements in the thin films with diffraction‐limited resolution. The CD signal shows a characteristic dependence on the film thickness, which supports a supramolecular origin of the strong chiral response of the copolymer. In particular, we demonstrate the discrimination between films of opposite chirality based on their ultrafast transient chiral response through the use of femtosecond broadband CD spectroscopy and a newly developed setup for transient CPL spectroscopy with 28 ps time resolution. A systematic variation of the enantiomeric excess of the chiral additive shows that the “Sergeants and Soldiers” concept and “Majority Rules” are not obeyed.


Differential Scanning Calorimetry (DSC)
F8BT and an F8BT:(+)-2 mixture (with 33 wt% of the chiral additive) were each dissolved in THF. The concentration of F8BT and the mixture were 12 mg mL −1 and the solutions were treated in an ultrasonic bath for 60 minutes. Afterwards, the solutions were filtered through 0.45 µm PTFE filters with a syringe. The solutions were dripped step-by-step into aluminum crucibles (Netzsch, 25 µL) and dried under a flow of nitrogen. The procedure was repeated until a mass of about 10 mg was reached. The pans were then further dried in a nitrogen atmosphere with less than 1% humidity for about 12 hours. Prior to the measurement, small holes were pierced into the lid, then the crucible and the lid (Supporting Figure 1.1) were pressed together and the exact mass was measured. DSC data were collected using a thermo-microbalance-based simultaneous TG-DSC instrument (Netzsch STA 449 C Jupiter). An empty aluminum crucible served as a reference. Two independent nitrogen gas flows (20 ml min −1 each) and an additional argon gas flow (20 ml min −1 ) were employed for purging. Multiple temperature segments were traversed and are summarized in Supporting Table 1.2. A fixed heating rate of 10 °C min −1 was used for the heating and cooling segments. The first segment went up to a temperature of 80 °C to evaporate residual THF. After cooling down to about 30 °C, the next segment went up to a temperature of 170 °C. According to Donley et al., the glass transition temperature of F8BT samples with different molecular weights (M n = 9255 kg mol −1 ) is in the range 135140 °C. [2] A temperature of 170 °C should therefore provide a well resolved signal for the glass transition and a non-destructive first scan for the F8BT:(+)-2 blend, as we observed beginning decomposition of the chiral additive above 200 °C. After cooling down to 30 °C, two heating cycles to 320 and 330 °C, respectively, were carried out, each one followed by cooling to 30 °C.

Atomic Force Microscopy (AFM)
Images of the topography of annealed F8BT and F8BT:(+)-2 thin films (with 9, 17 and 33 wt% of the chiral additive) were taken with an atomic force microscope (PSIA XE-100) in non-contact mode using a cantilever with a silicon tip. The resolution was 512 times 512 pixel for an area of 6  6 µm 2 , with a scan rate of 0.5 Hz. The AFM images were evaluated using the software package Gwyddion [3] (version 2.61) and visualization was carried out using OriginPro2022 (OriginLab Corporation). Forward and backward scans were averaged and afterwards background corrected.

Steady-State Absorption Spectroscopy and Microscopy
A Varian Cary 5000 spectrophotometer was employed to measure steady-state absorption spectra of macroscopic (5  5 mm 2 ) thin film areas using a slit width of 0.5 nm. CD spectra were recorded on the same spectrometer by adding a home-built assembly featuring polarizer -achromatic quarter-wave plate combinations covering the UVVis range, as described previously. [4] For systematic measurements of the chiral response for different weight ratios of the copolymer and the chiral additive, we also used an Applied Photophysics Chirascan instrument, which was kindly provided by the Ihmels group (Organic Chemistry 2, University of Siegen). Circular dichroism, circularly polarized luminescence and crossed-polarizer microscopy experiments of the thin film samples were performed on a recently established setup, which is built around a modified Olympus IX71 inverted microscope, providing images with a diffraction-limited resolution of about 400 nm as well as CD and CPL spectra integrated over the entire field of view. [4]

Ultrafast Broadband Transient Circular Dichroism and Absorption Spectroscopy
Transient circular dichroism and transient absorption spectra were recorded on a setup based on an amplified titanium:sapphire laser system (Coherent Libra USP-HE), as described previously. [4] The thin film samples, which were kept inside an aluminum cell under a constant flow of dry nitrogen, were excited at 320 nm and probed by broadband multifilament UVVis supercontinuum pulses (260700 nm), which were either alternately left and right circularly polarized (TrCD experiments) or linearly polarized (transient absorption measurements). The kinetic analysis of the TrCD data was performed using the program Tenua 2.1. [5] The rate constants in the mechanism were varied to arrive at an optimal description of the TrCD kinetics. The thickness of the polymer films was determined from the transient absorption kinetics using picosecond ultrasonics [6,7] assuming that the longitudinal sound velocity of the thin films is equal to that of the previously investigated closely related copolymer c-PFBT ( L = 2490 m s -1 ). [8] Error bars for the thickness were determined from an error propagation analysis considering the known uncertainty of  L [8] and the uncertainty of the period of the coherent acoustic phonon oscillation obtained from the fit.

Time-Correlated Single Photon Counting
A new setup for time-correlated single photon counting (TCSPC) was constructed to record transient photoluminescence decays. The samples were photoexcited by the second harmonic (410 nm) of a mode-locked titanium:sapphire oscillator (Spectra-Physics Tsunami, pulse duration 80 fs, 80 MHz repetition frequency). Tuning of the polarization and the laser pulse energy was achieved by the combination of a Glan-Taylor polarizer (Karl Lambrecht, MGTYS10) and a tunable zero-order halfwave phase retardation plate (Alphalas, PO-TWP-L2-25-UVIR). The photoluminescence of the samples was collected at an angle of 90° by a quartz lens, sent through a bandpass filter (Thorlabs, FB530-10, center wavelength 530 nm, FWHM 10 nm), and then refocused by another quartz lens onto a hybrid multialkali photodetector with a cathode diameter of 3 mm (Becker & Hickl, HPM-100-07). The signal pulses from the detector and the synchronization pulses of the laser (recorded by a fast photodiode, Becker & Hickl, PHD-400-N-SET) were fed into the inputs of a TCSPC module (Becker & Hickl, SPC-130IN) which was operated in reversed start-stop configuration.
For lifetime measurements, the samples were excited by vertically polarized laser light (0°) and the photoluminescence was recorded at the magic angle (54.7°) employing a wire-grid polarizer (Thorlabs WP25M-UB) in the photoluminescence detection path. For time-resolved CPL measurements, the 410 nm laser pulses were sent through a Hanle depolarizer to scramble the polarization prior to excitation. In the detection path, an achromatic broadband quarter-wave plate (Thorlabs AQWP05M-580) was inserted between the collecting lens and the polarizer. CPL kinetics were obtained by subtracting the decays of two consecutive TCSPC measurements (recorded for identical collection times) with the polarizer axis fixed at 0° and the fast axis of the quarter-wave plate set at either +45° or 45° with respect to the polarizer axis. An instrument response function (IRF) of 28 ps was determined from the laser scattering signal of a diluted suspension of colloidal silica nanoparticles (Merck, Ludox AM-40). The time constants and amplitudes of the thin-film photoluminescence decays were obtained from tail fits employing the FAST program (Edinburgh Instruments).
Additional TCSPC experiments with lower time-resolution were carried out using a pulsed UV-LED (Becker & Hickl, UVL-FB-270, 273 nm, 20 MHz repetition frequency) as the excitation source. For lifetime measurements, the output of the LED was vertically polarized (0°) by means of a wiregrid polarizer (Thorlabs WP25M-UB). For time-resolved CPL measurements, the samples were excited directly by the unpolarized output pulses of the LED. For these experiments, the time resolution was limited by the LED pulse width of 500 ps.

Supporting Note 2. Crossed-Polarizer Images of F8BT:(+)-2 Thin Films
In Supporting Figure 2.1 we present crossed-polarizer images of different F8BT:(+)-2 thin films. The upper row contains images of a pristine film (panel a) and an annealed (150 °C) film (panel b) spincoated from a chlorobenzene:chloroform mixture. The lower row shows similar images, but for films spin-coated from toluene. The black images in panels a and c indicate that the pristine (asdeposited) films do not turn the initially linear polarization of the light. This is consistent with the presence of only weak molecular optical activity due to the embedded chiral helicene-type additive. In contrast, the crossed-polarizer images of the annealed films in panels b and d show characteristic granular textures, which are consistent with the structures observed in the CD and CPL images (cf. Annealed thin film originally spincoated from a mixture of chlorobenzene and chloroform. c) Pristine thin film spin-coated from toluene. d) Annealed thin film originally spin-coated from toluene. Blends were deposited employing 33 wt% of the chiral additive. Images for a/b and c/d were recorded for identical conditions. The white scale bar in each image corresponds to a distance of 25 m.

Supporting Note 3. Chiral Response of Pristine and Annealed F8BT:(+)-2 Thin Films
In Supporting Figure 3.1 we present CD microscope images and spectra of F8BT:(+)-2 thin films spincoated from a chlorobenzene:chloroform mixture (1:9 v:v) in pristine form (panel a) and after annealing at 150 °C (panel b). Panels c and d show corresponding data, but for these thin films the spin-coating was performed from toluene. The images and spectra of the pristine films in panels a and c show no CD response, whereas the annealed films in panels c and d exhibit pronounced CD activity. showing the dissymmetry parameter g abs of a pristine F8BT:(+)-2 thin film spin-coated from a chlorobenzene:chloroform mixture (top) with the distribution of g abs values (middle) including a Gaussian fit (dashed red line), determined over the entire field of view, and corresponding spectra integrated over the entire field of view (210  160 m 2 , three panels at the bottom) displaying the optical density for left-and rightcircularly polarized light (OD L (black line), OD R (red line)), the CD spectrum (OD L -OD R , blue line) and the g abs spectrum (green line), with the thick red line indicating the spectral region selected by the bandpass filter (470 nm, FWHM 10 nm) used for CD imaging. b-d) Same as in panel a, but for an annealed F8BT:(+)-2 thin film spin-coated from a chlorobenzene:chloroform mixture, a pristine F8BT:(+)-2 thin film spin-coated from toluene, and an annealed F8BT:(+)-2 thin film spin-coated from toluene, respectively. The scale bar in each CD image corresponds to a distance of 25 m. In each case, 33 wt% of the helicene-type additive was employed.

Supporting
Supporting Figure 3.2 shows bulk steady-state absorption spectra (top) for LCP and RCP probing (dashed and solid lines, respectively), CD spectra (middle) and g abs spectra (bottom) for annealed thin films of F8BT:(+)-2 thin films spin-coated from a chlorobenzene:chloroform mixture (1:9 v:v, black lines) or from toluene (red lines), and also for a thin film of the pure helicene-type additive (+)-2 (blue lines). After deposition, all films were annealed at 150 °C. The F8BT:(+)-2 blends exhibit a strong chiral response. The large CD values arise from supramolecular chirality. In contrast, on the same OD scale, the thin film of the pure helicene-type additive (+)-2 does not show any appreciable optical activity, which suggests molecular chirality, i.e. the thin film of the chiral additive does not form a chiral supramolecular arrangement. Figure 3.2. Steady-state optical spectra for annealed F8BT:(+)-2 thin films spin-coated from a chlorobenzene:chloroform mixture (black, 1) or toluene (red, 2) and for an annealed thin film of pure compound (+)-2 (blue, 3). Top) Absorption spectra for LCP (dashed lines) and RCP detection (solid lines). Middle) CD spectra. Bottom) g abs spectra. Blends were deposited employing 33 wt% of the chiral additive.

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In Supporting Figure 3.3, we provide spectra for the pristine (= as deposited) thin films for comparison. In this case, the thin films of the blends do not show CD activity. Formation of a chiral supramolecular arrangement therefore requires annealing of the blends above the transition temperature.

Supporting Note 4. Invariance of CD Signals of F8BT:2 Thin Films Under Sample Rotation
Supporting Figure 4.1 shows CD microscope images and spectra of an F8BT film containing 33 wt% of an enantiomer mixture of the helicene-type chiral additive 2, with 10% enantiomeric excess of ()-2. Under these conditions, small domains of opposite chirality with high g abs contrast can be easily distinguished in the CD microscope, as indicated by the small red and blue regions appearing side-by-side in the overview microscope images in the top row of the figure. This property makes such films ideal candidates to check the invariance of the chiral response upon sample rotation, even for very small regions with length scales on the order of the diffraction limit. showing the optical density for left-and right-circularly polarized light (OD L (black), OD R (red)), the CD spectrum (blue) and the g abs spectrum (green), with the thick red line indicating the spectral region selected by the bandpass filter (470 nm, FWHM 10 nm) employed for CD imaging. b-d) same as in panel a, but for a rotation of the sample by 90°, 180° and 270°, respectively.

Supporting
In the second row, we zoom into a specific region of the film (indicated by a rectangle in each of the images in the first row). The selected part of the image features a prominent horseshoe-shaped red region of positive g abs . One camera pixel in each of these images represents a CD measurement for a 160 x 160 nm 2 area, and the estimated optical resolution is 400 nm (Abbe diffraction limit, with a numerical aperture of the 40x objective of 0.60 and a center wavelength of the bandpass filter of = 470 nm). Upon rotation of the sample in 90° steps, one can nicely follow the corresponding rotation of the horseshoe-shaped region. Most importantly, there are no changes in the sign and amplitude of the CD response of this region. Therefore, we conclude that contributions resulting from a combination of linear dichroism (LD) and linear birefringence (LB) in combination with any possible anisotropies of the CD microscopy setup can be safely excluded for length scales down to 400 nm. The finding is confirmed by the OD L , OD R , CD and g abs spectra in the bottom row, which all yield results which are invariant upon rotation.
In addition, Supporting Figure 4.2 shows area-integrated spectra for OD L , OD R , OD L OD R and g abs of an arbitrary area of an F8BT:(+)-2 thin film obtained using the CD microscope setup. There are virtually no changes in the CD response and the dissymmetry factor g abs upon turning the thin film sample, meaning that the response of the samples is invariant under rotation. Thus, we can again exclude contributions resulting from a combination of LD and LB effects in our sample in combination with any possible anisotropies of the CD microscope optics. We previously observed very similar behavior for thin films of the intrinsically chiral copolymer c-PFBT. Such a behavior is a hallmark of multidomain supramolecular liquid crystalline order with statistical orientation of the individual domains. [9,10] There, it was also shown that such samples do not change their CD response when the sample is flipped. 3, respectively. We start with the discussion of the first heating scan (top panels). An exothermic peak at 130 °C in the heat flow curve was attributed to the crystallization step from an amorphous glassy state to a nematic phase or rubbery state. [2] A Gaussian peak analysis provided a minimum in the first derivative at 122.3 and 121.6 °C for pure F8BT and the F8BT:(+)-2 blend, respectively, which corresponds to the inflection point of the heat flow. It was therefore assigned to the glass transition temperature T g . Most importantly, addition of the chiral helicene-type compound practically does not change the glass transition temperature of the polyfluorene copolymer. The melting range is indicated by the last local maximum in the heat flow curve. The zero crossing of the first derivative provides a melting temperature T m of 206.7 and 198.5 °C for pure F8BT and the F8BT:(+)-2 blend, respectively.

Supporting
A similar analysis was performed for the second heating scan (bottom panels). Here, we obtained values of T g = 138.1 (137.2) °C and T m = 204.2 (202.6) °C for pure F8BT (the F8BT:(+)-2 blend). We note that the T g values for the second heating scan agree well with the range 135140 °C reported by Donley et al. [2] However, for a direct comparison of F8BT and the F8BT:(+)-2 blend, the first heating scan is probably more reliable, as we noted some decomposition of the helicene-type additive starting at about 200 °C.

Supporting Note 6. Ultrafast Electronic Relaxation of F8BT:(+)-2 and F8BT:()-2 Blends
Here, we discuss the dynamic processes underlying the TrCD signals in more detail. Representative kinetic traces averaged over the low-wavelength band of the TrCD bleach region (340350 nm) of F8BT:(+)-2 and F8BT:()-2 were already presented in Figure 4d of the main manuscript. Both kinetic traces exhibit a fast decay over the first few picoseconds, which then slows down considerably, reaching a residual signal of about 15% at 1000 ps. These dynamics are directly correlated with the recovery of S 0 population, because the supramolecular TrCD response does not contain clear CD contributions from F8BT excited states. [4] Importantly, the fast decay of the TrCD signal at early times is not compatible with the intrinsic lifetime of the S 1 exciton state. We determined this independently using time-correlated single photon counting (TCSPC) at very low S 1 exciton densities (N(S 1 ) < 1  10 10 cm 3 ), where processes involving two S 1 singlet excitons cannot occur because of the large excitonexciton distance. Supporting

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The decay of the photoluminescence intensity is well described by a triexponential function: The lifetimes  i and corresponding percentage amplitudes A i were extracted from tail fits and are reported in Supporting Table 6.1. We also include fractional contributions to the photoluminescence intensity f i (with i = 1, 2 or 3) which are defined as The multiexponential character of the decay is not unexpected, because individual F8BT chromophores in blends with the helicene-type additive experience quite different environments with varying contributions of F8BTF8BT and F8BT2 interactions. The photoluminescence decays are dominated by two time constants: 119 and 286 ps for F8BT:(+)-2 and 132 and 304 ps for F8BT:()-2. The 1020 ps difference between the two films probably arises from slight variations in the film preparation conditions. The amplitude ratio is roughly 2:1 and the two components together amount to more than 99% of the total amplitude. The third time constant of about 1200 ps has only a contribution of 0.5% and could arise from a very small amount of a luminescing impurity in the polymer. The resulting average lifetimes obtained from eq. (S6.3) are 233 and 253 ps for the two different films. Note that this time scale is much slower than the initial decays observed in Figure 4d of the main manuscript, which are on the order of a few picoseconds.
Supporting Table 6   A more elaborate kinetic description is therefore required to describe the kinetics in Figure 4d of the main manuscript for the blends consisting of F8BT and the helicene-type chiral additives (+)-2 and ()-2. We utilize a simplified version of the kinetic model introduced recently for the relaxation of c-PFBT thin films: [4]     In the following, we briefly summarize the key steps of the model: [4] Initially, a higher-lying singlet exciton state, denoted as S x , is excited by the pump wavelength 320 nm. It decays with the rate constant k x =  x 1 to S 1 singlet excitons, which relax further to S 0 with the rate constant k 1 =  1 1 (step S6.4). We associate  1 with the average lifetime  determined from the TCSPC measurements and use an averaged value of 243 ps for the F8BT copolymer in both films (cf. Supporting Table 6.1). As discussed previously, the S x state can also directly dissociate into a charge pair state (CP, electron (e  ) plus hole (h + )) according to step S6.5 (rate constant k CPx =  CPx 1 ). [4,11] Beside unimolecular singlet exciton relaxation, there are also singletsinglet annihilation (SSA) channels, which are only efficient at high exciton number densities. One possibility is diffusive bimolecular SSA according to step S6.6. Here, two S 1 excitons diffuse along an F8BT chain and then react, once they reach a critical distance. This leads to the formation of a high-energy exciton species S n and an S 0 ground state. Alternatively, SSA between two S 1 excitons may also take place by nonradiative Förster resonance energy transfer (FRET). [12,13] However, our previous study on c-PFBT showed that this is only a negligible channel. [4] Therefore, for the simple model applied here, the FRET process is not included. The excited species S n can either decay to S 1 (k n =  n 1 , step S6.7) or produce a charge pair via step S6.8 with k CPn =  CPn 1 . Bimolecular recombination of the charge pair occurs on longer time scales (k rec =  rec 1 ) and is described by step S6.9.
Contributions of the chiral additive to the spectral dynamics can be neglected, because the spectral CD signatures of films of these helicene-type molecules are distinctly different and much weaker than those of chiral F8BT arrangements. [14] However, the electronic interactions between the chiral additive and F8BT in addition to the F8BTF8BT interactions might influence the relaxation time constants of F8BT. In addition, the overlap of the two absorption bands at the pump wavelength of 320 nm needs to be considered, because a fraction of the photons will be absorbed by the chiral additive and will not be available for the photoexcitation of F8BT. This can be evaluated by separating the steady-state absorption spectra of the blends into absorption contributions of the chiral additive and F8BT, leading to an effective absorbance of F8BT at 320 nm. Examples for this procedure are shown in Supporting Figure 10.1. The effective absorbance enters the determination of the initial F8BT exciton number densities, which were 1.12  10 19 cm 3 for F8BT:(+)-2 and 9.32  10 18 cm 3 for F8BT:()-2.
In Supporting Figure 6.2, the TrCD signals at short times (panel a) and long times (panel b) are displayed in terms of the absolute exciton number densities. The TrCD kinetics monitor the recovery of the S 0 ground state population and do not show contributions from electronically excited states.

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The experimental data for F8BT:(+)-2 (open blue circles) and F8BT:()-2 (open blue squares) therefore directly reflect the number density of F8BT in S 0 . The kinetic model given by steps S6.4S6.9 was implemented using the program package Tenua 2.1. [5] The kinetic parameters were optimized to simultaneously fit the TrCD kinetics of both films. Compared with the fits for the previously investigated c-PFBT system, [4] the rate constant k CPx had to be increased to account for the larger quantum yield for CP formation from S x . The rate constant k rec for CP recombination was slightly reduced to describe the saturation behavior of the kinetic traces at long times better.

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The kinetic model describes the changes in the S 0 population very well, both on short and long time scales, as shown by the solid blue lines (F8BT:(+)-2) and the dashed blue lines (F8BT:()-2). The fast recovery at early times (panel a) is predominantly due to diffusive S 1 S 1 exciton annihilation, where the S 1 exciton number densities are shown as solid and dashed red lines. The TrCD kinetics also provide information regarding the efficiency of CP formation from the highly excited S x and S n exciton states of F8BT. Taking the results from the best fit, we obtain a yield of 19% for charge pair formation from S x and about 6% for the same process in the S n state. The total yield of charge pair formation is therefore 25% and thus slightly larger than the 15% previously found in the case of c-PFBT thin films. Supporting Table 6.2 summarizes the parameters obtained from the optimized fit. Table 6

Dependence of CD Signals on Film Thickness
Supporting Figures 8.18.3 show additional microscopy data for the dependence of the circular dichroism on film thickness. The experiments were performed on the same three films already discussed in the main manuscript ( Figure 6) and serve to illustrate the spot-to-spot variation of the optical properties for the individual samples. In general, the average g abs values agree very well for all samples, so the overall changes in the chiral response are minor. We used the variation of the g abs values for the different film regions to estimate error bars in Figure 6e of the main manuscript.
In some regions of the films, however, there is a distinct increase of the width of the g abs distributions, most prominently visible in Supporting Figures 8.2a and 8.3c. There are considerable variations in g abs , as indicated by the fine mosaic structure, where bright and dark spots with submicrometer spacing indicate substantial changes in the dissymmetry factor on small length scales, whereas for other films this distribution is more uniform. We are currently performing systematic studies to identify the reason for these differences, which could for instance originate from local variations of the annealing temperature across the film area during film preparation.

Supporting Note 9. Reflectivity Correction for the Dissymmetry Parameter g abs
Schulz et al. presented a formula to approximately account for reflection losses at thin films. [15] Their expression is derived from Stenzel's equation 7.9a for normal incidence on a thick slab neglecting interference effects and multiple reflections. [ Consequently, the corrected dissymmetry factor is obtained as: Note that their expression is for a free-standing slab of material, which is not the experimental situation in our case. Because of the substantial difference in the refractive indices (n 1  2.15 at the peak of the g abs spectrum of F8BT, [17] n 2  1.0 for air) the correction based on eq. (S9.4) will considerably overestimate the real correction, as it considers two interfaces of the copolymer thin film with air, whereas in our case we have the interfaces aircopolymer, copolymerglass and glassair. In any case, we obtain a value of 0.124 for the logarithmic correction term. As an example, for the three data points of the measurements dealing with the thickness dependence (violet solid circles in Figure 6e of the main manuscript), the corrected (uncorrected) absolute g abs values for the three films are then, in increasing order of thickness: 0.408 (0.216), 0.598 (0.381) and 0.556 (0.443). Even after the correction, the g abs value of the thinnest film is therefore substantially smaller than the g abs values of the thicker films.

Supporting Note 10. CD and AFM Experiments for Different F8BT:(+)-2 Blends
Supporting Figure 10.1 displays results from a systematic study of the chiral induction in the polyfluorene copolymer F8BT as a function of the weight percentage of the chiral additive (+)-2 using identical spin-coating and annealing conditions. The steady-state CD spectrometer (Applied Photophysics Chirascan) used in this experiment was kindly made available by H. Ihmels (Organic Chemistry 2, University of Siegen). Employing 9 wt% of the additive (panel a) results in a peak CD signal of 0.025 (830 mdeg) and a peak g abs value of 0.05. An increase of the chiral response is observed when 17 wt% of (+)-2 are used (panel b). Here, we obtain a peak CD value of 0.161 (5300 mdeg) and peak g abs value of 0.43. Using 33 wt% of the chiral additive leads to a further increase of the CD signal to 0.203 (6700 mdeg), whereas the peak g abs value is similar (0.39). The results show that already 17% of the helicene-type compound (+)-2 are sufficient to induce a substantial chiral response. The experiment also gives a good impression, how F8BT and the chiral additive contribute to the steady-state absorption spectrum of the blend (red and blue dashed lines in the top panels of Supporting Figure 10.1). To estimate the absorption contributions of (+)-2 and F8BT in the blend, a scaled spectrum of a pure thin film of (+)-2 (blue dashed line), which we measured previously, [14] was subtracted from the spectrum of the blend (black solid line), so that the characteristic peak of (+)-2 at 369 nm disappeared in the resulting F8BT spectrum (red dashed line).
Measurements of the absorption spectrum of pure F8BT show a smooth minimum in this wavelength range.
Supporting Figure 10.1. Chiral induction in the copolymer F8BT as a function of the weight percent of the chiral helicene-type additive (+)-2. a) For 9 wt% of (+)-2: Top panel: Steady-state absorption spectrum of F8BT:(+)-2 (black solid line), scaled steady-state absorption spectrum of a pure (+)-2 thin film (blue dashed line) and the difference between these two spectra (red dashed line), which should approximate the pure F8BT absorption in the blend; middle panel: CD spectrum, bottom panel: g abs spectrum. b) and c) Same as in a, but for 17 wt% (+)-2 and 33 wt% (+)-2, respectively. The values for the film thickness in panels a, b and c are 141, 120 and 235 nm, as measured by coherent acoustic phonon spectroscopy.

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In addition, we carried out AFM and CD microscopy experiments for annealed pure F8BT and also F8BT:(+)-2 thin films with varying amount of the chiral additive (9, 17 and 33 wt%) to check for a possible correlation between the surface morphology and the local CD activity of the thin films. The results are shown in Supporting Figure 10.2. The AFM images (top row) of the thin film surfaces show extended structures which could correspond to larger areas of fibrillar aggregates reported previously by Lakhwani and Meskers. [10] For the thin film with 33 wt% of the chiral additive, these structures appear to be more curved and elongated, with higher and lower regions being connected over longer distances. Values for the mean roughness S a (RMS roughness S q ) were 3.02 (3.86), 5.88 (7.68), 4.54 (5.71) and 5.58 (6.76) nm for F8BT thin films with 0, 9, 17 and 33 wt% of the chiral additive, respectively. Thus, the roughness of the blends does not depend systematically on the amount of the chiral helicene-type additive in the film. Figure 10.2. Comparison of atomic force microscopy and CD microscopy images for F8BT:(+)-2 films with different amounts of chiral additive. a) 0 wt% of the chiral additive (+)-2: AFM image with ca. 12 nm lateral resolution (top row), microscope image for the dissymmetry parameter g abs with about 400 nm optical resolution (second row), histogram of g abs values (green) with a Gaussian fit (dashed red line), determined over the entire field of view (third row), and spectra (bottom row) integrated over the entire field of view (210  160 m 2 ) showing the optical density for left-and right-circularly polarized light (OD L (black), OD R (red)), the CD spectrum (blue) and the g abs spectrum (green), with the thick red line indicating the spectral region selected by the bandpass filter (470 nm, FWHM 10 nm) employed for CD imaging. b-d) same as in panel a, but for thin films with 9, 17 and 33 wt% of the chiral additive, respectively.

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In contrast to the AFM images, which are sensitive to the surface morphology, the CD images monitor the chiral optical response of the complete film in transmission. The CD microscope provides a diffraction-limited resolution of ca. 400 nm, whereas the AFM images shown here have a lateral resolution of about 12 nm. The CD microscopy images are therefore considerably more blurred than the AFM images. Taking this blurring into account, the larger structures observed by AFM and CD microscopy appear to be quite similar. The size of these larger domains is in the range 600900 nm, similar to those seen in Supporting Figure S4.1.